Fused silica diaphragm module for high temperature pressure transducers

ABSTRACT

A high temperature pressure transducer and sensing apparatus to determine the deflection of the transducer diaphragm is disclosed. The pressure transducer utilizes a fused silica diaphragm (12) which is illuminated at selected locations by a coherent laser source (52) via optical fibers (38, 46). The light reflected by the diaphragm (12) forms interference fringe patterns which are focused by gradient index rod lenses (36) on the ends of optical fibers (40, 48) for transmission to a fringe counting circuit (54). By digital techniques, the fringe count is converted into a determination of diaphragm deflection.

The invention described herein was made in the performance of work underNASA Contract No. NAS 3-23712 and is subject to the provisions ofSection 305 of the National Aeronautics and Space Act of 1958 (72 Stat.435; 42 U.S.C. 2457).

TECHNICAL FIELD

The present invention relates to high temperature pressure transducers,and more particularly to a high temperature pressure transducer thatutilizes a fused silica diaphragm and associated fiber optic sensingapparatus to determine the deflection of the diaphragm.

BACKGROUND ART

Accurate pressure measurements in high temperature applications, such asin the gas path of an aircraft engine, are required in order to monitorand improve the fuel efficiency, performance, and reliability of theengine. Gas path pressure measurements in severe environments havetraditionally been performed through the measurement of the deflectionof metallic diaphragms. The resulting mechanical deflection of thediaphragm is converted into an electrical signal by several approaches.One method utilizes a resistive strain gage mounted to the center of thediaphragm. Another method utilizes the change in capacitance between themoving diaphragm and a fixed reference surface. Both of these approachesproduce acceptable results at relatively low temperatures, however, attemperatures in excess of 500° C., the creep of the metallic diaphragmaccelerates which results in a long-term drift of the pressuretransducer output signal versus pressure calibration curve. In addition,it has been found that hysteresis in this calibration curve may becomesignificant when these pressure transducers are operated at these hightemperatures.

In order to reduce or eliminate the undesirable creep and hysteresiseffects exhibited by metallic diaphragms at high temperatures, alternatediaphragm materials with improved high-temperature properties must beutilized. For example, various types of glasses and glass ceramics haveexcellent dimensional stability and these materials can replace metal asthe material for pressure transducer diaphragms. Unfortunately, thehardness and rigidity of these materials, along with their inherentbrittleness, dictate a diaphragm design that results in a smallerdeflection with pressure than the deflection achievable with metallicdiaphragms. These smaller deflections, in turn, necessitate the use ofsensing techniques having significantly increased sensitivity so thatthe deflections can be measured. Such icreased sensitivity allows themeasurements to be affected by dynamic vibration and temperature changeswhich may result in inaccurate measurements of diaphragm deflection.

Because of this, it has become desirable to develop a diaphragm-typepressure transducer and associated diaphragm deflection sensingapparatus that can be used in a high temperature environment, issensitive to relatively small diaphragm deflections, and is unaffectedby dynamic vibration and temperature changes.

SUMMARY OF THE INVENTION

The present invention solves the aforementioned problems associated withthe prior art and other problems, by providing a fused silica diaphragmassembly and associated fiber optic diaphragm deflection detectingapparatus. The diaphragm assembly is comprised of a fused silicadiaphragm optically contacted to a fused silica platform. A first pairof transmit and receive optical fibers is positioned in the center ofthe gap between the diaphragm and the platform and a second pair oftransmit and receive optical fibers is similarly terminated in the gapbut is offset from the first pair. A coherent laser source illuminatesthe bottom surface of the diaphragm via both transmit optical fibers.The light reflected by the bottom surface of the diaphragm causes aninterference fringe pattern to be created which is intercepted by thereceive optical fibers. Through photodetectors and a fringe countingcircuit, an interference fringe count is made which, in turn, isutilized to determine the amount of diaphragm deflection.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE of the drawing is a front elevation view schematicallyrepresenting the apparatus of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing where the illustration is for the purposeof describing the preferred embodiment of the present invention, and isnot intended to limit the invention hereto, the FIGURE illustrates amodular fused silica diaphragm assembly 10 and associated apparatus 30,shown schematically, to measure diaphragm deflection. The diaphragmassembly 10 is comprised of a fused silica diaphragm 12 opticallycontacted to a fused silica platform 14. The optical contact between thediaphragm 12 and the platform 14 requires a surface flatness of λ/10.Such surface flatness can be achieved and results in a true molecularbond between the diaphragm 12 and the platform 14.

The fused silica diaphragm 12 is typically formed from a solid discwhich may be drilled or etched to the proper depth to obtain the desireddiaphragm thickness. The diaphragm 12 and the platform 14 are formedfrom identical material, fused silica, and require no external supportstructure, thus minimizing thermal stresses. The platform 14 has anaperture 16 provided therethrough which terminates in the gap 18 locatedbetween the top reference surface 20 of the platform 14 and the bottomreference surface 22 of the diaphragm 12. A quartz tube 24 with a tipoffconstriction 26 can be fused to the bottom 28 of the platform 14 topermit the introduction of a reference pressure into the gap 18 throughthe tube 24 and the aperture 16. After the reference pressure has beenintroduced into the gap 18, the constriction 26 can be tipped-off toseal the reference pressure port.

The apparatus 30 utilized for measuring diaphragm deflection utilizes a"fringe-counting" technique to measure such deflection. With thisapparatus 30, two polished blind bores 32, 34 are provided in the topreference surface 20 of the platform 14. Blind bore 32 is located nearthe center of the top reference surface 20 and blind bore 34 ispositioned adjacent the junction of the top reference surface 20 and theinner edge of the diaphragm 12. A gradient index rod lens 36 is providedin each blind bore 32, 34. A single mode optical fiber 38 and amultimode optical fiber 40 pass through the platform 14 and interconnectthe gradient index rod lens 36 provided in the blind bore 32 to a 3 dbcoupler 42 and a photodetector 44, respectively. Similarly, a singlemode optical fiber 46 and a multimode optical fiber 48 pass through theplatform 14 and interconnect the gradient index rod lens 36 provided inthe blind bore 34 to the 3 db coupler 42 and a photodetector 50. Acoherent laser source 52 is connected to the input to the 3 db coupler42. The outputs of the photodetectors 44, 50 are connected to a fringecounting circuit 54 containing state-of-the-art devices, whose operationwill be hereinafter described. The output of the counting circuit 54 isconnected to an appropriate readout device 56.

Operationally, light from the laser source 52 is transmitted to the 3 dbcoupler 42 where it is divided and delivered to the gradient rod indexlens 36 provided in each of the blind bores, 32, 34 via the single modeoptical fibers 38, 46, respectively. The gradient rod index lenses 36project collimated light beams onto the bottom reference surface 22 ofthe diaphragm 12; the collimated light beam emanating from the gradientrod index lens 36 provided in blind bore 32 intercepting the bottomsurface of the diaphragm near the center thereof, and the collimatedlight beam emanating from the gradient rod index lens 36 provided inblind bore 34 intercepting the bottom surface of the diaphragm at alocation offset from the center thereof. The foregoing collimated lightbeams are partially reflected by the bottom reference surface 22 of thediaphragm 12 and the top reference surface 20 of the platform 14 causingthe production of interference fringe patterns on each gradient indexrod lens 36. These interference fringe patterns are focused by thelenses 36 onto the ends of the multimode optical fibers 40, 48. As thediaphragm 12 deflects, the interference fringe patterns moveunidirectionally across the input face of the multimode optical fibers40, 48. The direction of movement depends upon whether the diaphragm 12is deflecting toward or away from the top reference surface 20 of theplatform 14. The resulting output current of the photodetectors 44, 50has the same cosine squared time dependence with fringe position as doesthe spatial light intensity distribution across the fringes.

The fringe counting circuit 54 is comprised primarily of an up/downcounting device controlled by logic gates. By the appropriate choice ofthe trigger threshold, one of the photodetectos 44, 50 can be utilizedas an input to the up/down counting device through the logic gates. Eachbright-dark fringe pair of the interference fringe pattern is dividedinto four parts and the logic gates generate digital "high" signals anddigital "low" signals which correspond to individual counts having λ/8precision, wherein is λ is the laser wavelength. The output of theremaining photodetector 44 or 50 is utilized to determine whether thecounting device is to add or subtract the signals received from thefirst photodetector. In order to accomplish the foregoing, the outputsof the photodetectors 44, 50 must be out of phase. The decision to addor subtract the signals from the first photodetector is dependent uponwhether the outputs of both photodetectors 44, 50 are simultaneouslyincreasing or decreasing. The up/down counting device thus maintains arunning total of the interference fringe count.

The magnitude of the interference fringe count, m, is directlyproportional to the deflection, Δh, of the diaphragm 12 through thefollowing equation:

    2Δh=mλ

Thus, by digitally determining the fringe count, the deflection of thediaphragm 12 can be sensed and the amount of deflection can beaccurately determined. Inasmuch as digital techniques are utilized, nosignal conditioning or analog processing is required. In addition, sincethe interference fringe pattern is formed only at the gradient rod indexlenses 36, temperature and/or vibration in the multimode optical fibers40, 48 cannot affect the fringe pattern Furthermore, relative opticalphase shifts induced by the single mode optical fibers 38, 46 becomeunimportant inasmuch as such phase shifts affect the reflected beamsfrom the bottom reference surface 22 of the diaphragm 12 and the topreference surface 20 of the platform 14 identically. Phase preservationis not required for light to be conducted to the photodetectors 44, 50via the multimode optical fibers 40, 48 which act as a conduit for thelight emanating from a bright fringe or no light from a dark fringe.

Performance-wise, the inherent dynamic range of the foregoing system ishigh because the fringe count, m, changes in direct proportion to thedeflection, Δh, of the diaphragm 12. The displacement sensitivity ofthis approach is λ/8, which for red light corresponds to 0.09 μm.Assuming the diaphragm 12 has a 40 mm diameter and a 0.6 mm thickness,the minimum detectable pressure which corresponds to the foregoingdisplacement sensitivity is approximately 3.5 KP_(a) which is about 0.4%of the maximum allowable full-scale pressure of 690 KP_(a). Suchsensitivity is equivalent to many commercially available strain gage orcapacitive pressure transducers, however, such transducers cannottolerate the significantly higher operating temperatures which have noadverse effect on fused silica devices. And lastly, it should be notedthat the fused silica diaphragm assembly 10 is also inherently linearsince the deflection of the diaphragm is linear with pressure and theinterference fringe order is linear with diaphragm deflection.

Certain modifications and improvements will occur to those skilled inthe art upon reading the foregoing description. It will be understoodthat all such improvements and modifications have been deleted hereinfor the sake of conciseness and readability, but are properly within thescope of the following claims.

I claim:
 1. A high temperature pressure transducer comprising a basemember, a diaphragm member connected to said base member, said basemember and said diaphragm member being formed from a fused silicamaterial, and a gap located between said base member and said diaphragmmember allowing the deflection of said diaphragm member in response tovariations in the pressure applied thereto wherein said base member andsaid diaphragm member are molecularly bonded.
 2. The high temperaturepressure transducer as defined in claim 1 further including means forintroducing a reference pressure in said gap.
 3. The high temperaturepressure transducer as defined in claim 1 wherein a portion of saiddiaphragm member is offset from said base member to form said gaplocated between said base member and said diaphragm member.